Many steps ranging from a substrate manufacturing process to an element forming process are present in a period from slicing a single-crystal ingot of, e.g., semiconductor single crystal silicon (hereinafter, sometimes referred to simply as “a silicon”) to obtain a substrate to manufacturing a semiconductor device. As one of such steps, there is a heat treatment step. This heat treatment step is performed for the purpose of forming a defect-free layer at a substrate surface layer, forming a gettering layer based on formation of oxide precipitates, forming an oxide film, diffusing an impurity, and others, and it is a very important process.
As a heat-treating furnace used at such a heat treatment step, e.g., a diffusion furnace used for oxidation or impurity diffusion (an oxidation/diffusion apparatus), a vertical heat-treating furnace that performs a heat treatment with respect to a plurality of substrates at the same time while horizontally holding the substrates is mainly utilized with an increase in diameter of the substrates (see, e.g., Patent Literature 1). In the vertical heat-treating furnace, a heat treatment boat configured to hold a plurality of substrates is usually adopted. Such a heat-treating furnace that performs a heat treatment with respect to a plurality of substrates at the same time is called a batch type heat treatment apparatus.
FIG. 3 is a schematic explanatory view showing an example of a batch type vertical heat treatment apparatus. A reaction chamber 31 of a heat treatment apparatus 30 is mainly constituted of a reaction tube 32 and a flange body 33 provided under the reaction tube. It is to be noted that the reaction tube is made of silicon carbide (SiC) having high heat resistance, and the flange is made of quartz. A heat treatment boat 34 is disposed in the reaction chamber, and a plurality of substrates 35 held by this heat treatment boat are heated by a heater 36 provided around the reaction chamber. Further, an atmospheric gas supplied from a gas supply tube 37 is introduced into the reaction chamber via a gas introduction tube 38, and the gas is flowed from above the heat treatment apparatus to pass around the substrates and is discharged to the outside from a gas discharge tube 39.
As a material of this gas introduction tube 38, silicon carbide is used. That is because, when the gas introduction tube 38 is made of quartz, performing a heat treatment at a high temperature of approximately 1250° C. causes thermal deformation of the gas introduction tube, resulting in a problem that the introduction tube comes into contact with the heat treatment boat 34 to be destroyed. Therefore, in a heat treatment apparatus that performs high-temperature heat treatments at 1250° C. or above, a gas introduction tube made of silicon carbide is exclusively used.
Such a gas introduction tube 38 is connected to a gas port portion 41 at a connecting portion 40, and the gas port portion is connected to the gas supply tube 37 connected to a non-illustrated gas supply source. It is to be noted that, since the gas port portion has a complicated configuration and is hardly fabricated by using silicon carbide, quartz that facilitates fabrication is adopted. At this time, although the atmospheric gas to be used differs depending on a purpose of a heat treatment, H2, N2, O2, or Ar is principally utilized. Furthermore, in case of impurity diffusion, each of these gases is used as a carrier gas to introduce an impurity compound gas.
However, when the above-described batch type heat treatment apparatus is utilized to perform a heat treatment for a silicon wafer as a substrate under an inert gas atmosphere of, e.g., Ar, there occurs a problem that haze is generated on the silicon wafer.